The present invention relates generally to steam turbine calibration. Specifically, the invention relates to systems and methods of installing and calibrating packing rings (i.e. turbine packing) which surround turbine shafts.
When installing turbine packing, it is first necessary to load the packing in the holders with the original springs installed in the lower and upper halves. With the rotor removed and the upper half of the casings overturned to allow access to the upper half, segments for each ring of packing are installed into the packing holders of the upper and lower halves. The springs are difficult to install and remove, due to the spring stiffness necessary to hold the weight of the packing segments in their close clearance position during operation, and often require significant force to install and remove, depending on the size and weight of the packing segments.
The springs are necessary in order to hold the packing segments in their operational position against the hooks of the packing holders and to enable the measurements for the amount of material to be machined off of the packing segment butts in order to create radial clearances to accommodate thermal expansion of the packing rings during operation.
The upper half packing is measured to determine the exact location of the upper half packing pin slots. The pins are necessary in order to keep the packing from rotating during operation, and from falling out during assembly and disassembly of the upper half. All segments are then unloaded and the butts and pin slots machined for the specific radial and circumferential clearances. After machining, the segments are again reloaded in the packing holders with springs in the upper and lower halves and the pins installed in the upper halves. This necessary procedure is very time consuming, since there may be from 10 to 100 rings or more of packing to install.
Non-retractable packing segments have associated packing springs behind them (against the packing holder) during operation. The retractable packing segments have associated packing springs situated circumferentially between the packing segments (i.e. between packing butts). Retractable packing replaces the spring normally located behind the conventional packing ring segments with coil springs located in holes machined in the butts of the packing segments.
The coil springs in retractable packing cause the packing segments to move radially away from the shaft (in operation), taking up the amount of space between the back side of the packing ring and the packing holder. As steam flow increases, the steam pressure behind the packing overcomes the spring and friction forces, causing the packing to close (i.e. move towards the rotor in a close clearance position). The packing segments will then be operating with design clearance.
With “sensitized packing”, sensitized springs are positioned radially behind the packing segments with one spring circumferentially between the top segments. The packing “floats” on these springs. The upper packing ring segments are isolated from the lower segments by keys at the horizontal joint to minimize necessary spring forces. Sensitized packing rings have the ability to provide a seal with reduced rubbing forces when compared to conventional rings.